Sequence stratigraphy and geomodeling of the upper Pennsylvanian Canyon and Cisco Formations in SACROC Field, West Texas : insights into the Late Paleozoic glaciation

Alnazghah, Mahmoud Hasan
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The Late Paleozoic has long been recognized as a time of extensive glaciation. Recent work has suggested that the Late Paleozoic represents greenhouse setting, with limited to no ice caps. The nature of the climate, greenhouse vs. icehouse, plays a major control on the observed stratigraphic architecture. Compared to the layered architecture of the greenhouse climate, where accommodation is filled to spill, the stratigraphic architecture of icehouse climate is dominated by complex geometries, discontinuous facies belts and frequent subaerial exposure surfaces and diastems. Such extreme heterogeneity represents a challenge in modeling subsurface reservoirs, which are inherently poorly sampled. A high resolution sequence stratigraphic framework was constructed for the Late Pennsylvanian units in the Scurry Area Canyon Reef Operators Committee (SACROC) Field using an extensive dataset including more than 7000 ft of core, 650 wireline logs, and 3D seismic volume. The stratigraphic framework was used to approximate water depths of lithofacies and subsequently used to construct a eustatic sea level curve for Late Pennsylvanian time. Several modeling methods were tested to investigate the best approach to represent complex icehouse carbonate systems in the subsurface. The sequence stratigraphic framework of the Late Pennsylvanian shows a transition from layered-cake stratigraphy during the deposition of the Missourian Canyon Formation into a wedding-cake architecture during the deposition of the Virgilian Cisco Formation. This transition and the refined eustatic curve are indicative of an increased icehouse conditions that peaked during the Virgilian. Periodicity was estimated from number of cycles and time to be 100 k.y. and 343 k.y. for the Missourian and Virgilian respectively, suggestive of short- and long-term eccentricity signal. Modeling results showed that pure stochastic methods failed to capture the complexity expected within icehouse carbonate platforms. On the other hand, multiple-point statistics algorithm, which is guided by conceptual models based on outcrop and modern analogs, generated the most geologically sound models. The refined eustatic curve for the Late Pennsylvanian is useful in improving and constraining paleoclimate, which can be employed in future climate simulations. The modeling efforts presented here are one of the few to capture the expected 3D heterogeneity of icehouse carbonate platforms, which is key for developing similar hydrocarbon reservoirs.